The word "sputtering" is used to describe the process in which a cathode (target) is subject to ion bombardment in a vacuum plasma.
Sputtering is one of the most convenient and economical ways to deposit thin films on a substrate. The rate of deposition is comparable to other deposition techniques. Flexibilities in terms of controlling the process parameters, which determine the quality of the deposited film, are excellent. In addition, sputtering techniques also can be used to sputter-etch, for example, etching of wafers before metalization.
The simplest type of sputtering device is composed of a vacuum chamber with an anode-cathode assembly, in which, the chamber is pumped down to a very low pressure. Then, a pure gas, most commonly argon, is introduced into the system. Finally, a high voltage is applied across the anode and cathode of the system. Upon application of that high field, the gas breaks down and a glow discharge begins. The most important region in a glow discharge is the cathode fall region. In this area, bombarding ions are produced by electron-neutral gas molecule collisions. On the other hand, secondary electrons are emitted due to the bombardment of the target (cathode) surface by the gas ions; thus, the sputtering process continues. In the case of DC sputtering, a grounded metallic chamber also is used as the anode of the system.
The DC sputtering process can be used only to sputter and etch conductive materials. Because DC cannot pass through an insulating material, a radio frequency (RF) source is used to sputter and etch insulators. The industrially allocated frequency for this purpose is 13.56 MHz. In the case of RF sputtering, the source (RF generator) is connected across the target and grounded vacuum chamber through a matching network, as shown in FIG. 1(A). Upon application of the RF field, a glow discharge can be initiated. Then, during the negative half cycle of the field at the target, ions are attracted toward the target, causing sputtering. However, these bombarding ions are also responsible for the emission of electrons from the target surface. As a result, the insulating target material becomes positively charged. This charge is neutralized during the next cycle when the target becomes positive and attracts electrons from the glow discharge.
Although an RF glow discharge is initially produced by using a symmetrical sinusoidal output of the RF generator, in an assymmetrical sputtering system there appears a negative self-bias at the target. This can be explained as follows: due to the alternating characteristics of the RF field, the RF voltage cannot recognize which electrode is anode and which electrode is cathode. However, if the target surface area is much smaller compared to the chamber surface area, then this asymmetry between the surface area of the two electrodes forces the glow discharge to establish a negative self-bias at the target with respect to the ground, or, more precisely, with respect to the plasma potential. This is shown in FIGS. 2(A) and 2(B).
The establishment of negative self-bias or DC offset at the target is very important in an RF excited sputtering process because this self-bias determines the energy of the bombarding ions. Thus, DC offset can be considered as a counterpart of cathode fall voltage in the case of DC sputtering. The relationship between the amount of negative self-bias and the area ratio of the two electrodes is not exactly known although it is known the smaller the size of the targer the higher the DC offset.
An equivalent electrical circuit representing an RF glow discharge system is shown in FIG. 1(B) where the following is a description of the circuit components:
(1) Cp and Lp: Plasma capacitance and inductance PA1 (2) Cst: space charge sheath - target capacitance PA1 (3) Csw: space charge sheath - chamber wall capacitance PA1 (4) Rs=Rst+Rp+Rsw=resistive part of the system PA1 (5) Ctw: fixed capacitance due to target mounting on the wall PA1 (6) Cd: distributed or stray capacitance
Values of the different circuit components (except Ctw and Cd) primarily depend on target and vacuum chamber size, target material, argon gas pressure, RF power and external magnetic field.
The characteristics of an RF sputtering system can be summarized as follows:
(a) DC offset at the target is established in an asymmetrical RF excited sputtering system. The amount of this DC offset determines bombarding ion energy and so the sputtering rate.
(b) RF glow discharge impedance is mostly due to its capacitance.
(c) Capacitance due to target amount and stray field act in parallel to the glow discharge impedance. Since they are acting in parallel, the sputtering system impedance due to its reactive part cannot be higher than the impedance due to external (target mount and distributed) capacitance. Furthermore, this external capacitance often contributes a much lower impedance compared to the glow discharge impedance and, thus, a significant amount of RF power is wasted.
The area ratio problem in an RF sputtering system is unavoidable due to the physical nature of the RF excited glow discharge. This prevents achieving high sputtering yields from a large target. Table 1 below shows how poor the existing RF sputtering process is when compared to DC sputtering
TABLE I ______________________________________ SYSTEM DESCRIPTION Magnetically enhanced sputtering Target size = 5" .times. 15" = 75 square inches Power = 67 watts/sq. inch Vacuum chamber size 2' dia .times. 2' high ______________________________________ Power Source Target Sputtering Rate Angstroms/min ______________________________________ DC Cu 12,000 DC Al 6,000 RF Al.sub.2 O.sub.3 300 RF SiO.sub.2 700 ______________________________________ Source: VacTec Systems, Inc., Boulder, Colorado; SCP and Solid State Technology, December 1966, pp. 31-36; and applicant's own experience.
In order to increase the sputtering rate it is necessary to produce a sufficiently high negative self bias or DC offset at the target. For example, if there exists a 500 V DC offset at the target, the ions may accelerate and bombard the target with 500 eV energy. The results of sputtering of various targets using 500 eV ions are shown in Table II below. These ions were produced from an ion beam source.
TABLE II ______________________________________ Ion Energy = 500 eV Target Sputtering Rate, Angstroms/min ______________________________________ Cu 870 Al.sub.2 O.sub.3 129 SiO.sub.2 400 ______________________________________ Source: Ion Tech, Inc., Fort Collins, Colorado.
In Table I, for example, the sputtering ratio of Al.sub.2 O.sub.3 to Cu is 300/12,000=0.025. However, in comparison, this same ratio using the data from Table II, i.e., 129/870=0.148, is 6 times higher than the previous ratio. This comparison also indicates that in an RF excited glow discharge, if one is able to increase the bombarding ion energy to 500 eV then the sputtering rate should increase six times while using a large target.
In the case of DC reactive sputtering, wherein a metallic target is sputtered in a partial atmosphere of a gas such as O.sub.2, the oxide of the metal of the target tends to build up on the target. Since this oxide is typically non-conductive, the problems of target arcing become severe.
As can be appreciated from the foregoing a method and apparatus is needed for sputtering dielectric materials or for reactive sputtering wherein the sputtering rate is substantially increased with respect to conventional RF sputtering.
A number of techniques, other than conventional RF sputtering are described in the prior art for sputtering dielectrics or for reactive sputtering and have met with varying degrees of success. Some of these are as follows.
In U.S. Pat. No. 3,594,295 (copy submitted herewith) an RF sputtering apparatus is disclosed for sputtering dielectric materials wherein potentials of opposite polarity are applied to the target, the potentials being alternated at radio frequencies. Thus, one-half of the target is negative and thus sputtered while the other half of the target is positive and repels positive ions. Ion bombardment during this interval of sputtering deposits a positive charge on the negative one-half of the non-conducting target that is sputtered. In order that this positive charge does not subsequently repel and inhibit further sputtering, the opposite electrode that was previously sputtered is positive and attracts electrons sputtered from the other half of the target. This effectively neutralizes the positive charge acquired previously.
U.S. Pat. No. 3,464,907 (copy submitted herewith) is directed to a triode sputtering device wherein the first pulse generator 40,24 applies pulses across an anode 21 and a cathode filament 20 to thus establish plasma within the chamber. A second pulse generator 40,54 applies pulses between the target 27 and an anode 21 to render the target negative with respect to the anode to thus effect sputtering of the target, the sputtered material being deposited on a substrate 35. The apparatus is usable with either metallic, insulative or targets made of other materials.
U.S. Pat. No. 4,131,533 (copy submitted herewith) discloses enhancement of the resputtering rate in RF sputtering apparatus where the anode shield is isolated from ground potential.
U.S. Pat. No. 4,046,659 (copy submitted herewith) discloses a method of reducing arc formation in a reactive, magnetron sputtering system where an AC potential is applied between the anode and cathode.